SuccinateUbiquinone Oxidoreductase is a large membrane protein complex which introduces reducing equivalents from succinate into the mitochondrial respiratory chain. It comprises succinate dehydrogenase of the Krebs cycle (flavoprotein and ironsulfur protein subunits) as well as two hydrophobic membrane proteins, one of which carries protoheme. It is evolutionarily related to fumarate reductase present in many bacteria. We have crystallized it in an orthorhombic form diffracting to 3.5 E.